Using 64-bit Custom Actions
On 64-bit operating systems, Windows Installer can call custom actions that are compiled for 32-bit or 64-bit systems. A 64-bit custom action based on Scripts must be explicitly marked as a 64-bit custom action by adding the msidbCustomActionType64BitScript bit to the custom action numeric type in the Type column of the CustomAction Table. Custom actions based on Executable files or Dynamic-Link Libraries that are complied for 64-bit operating systems do not require including this additional bit in the Type column of the CustomAction Table.
For more information, see 64-Bit Custom Actions.